Cathodic sputtering is widely used for the deposition of thin films, or layers, of material onto desired substrates. The sputtering process employs gas ion bombardment of a target material having a face formed of a material that is to be deposited as a thin film, or layer, on the given substrate. Ion bombardment of the target material not only causes atoms or molecules of the target material to be sputtered, it imparts considerable thermal energy to the sputter target assembly, causing the target to expand and contract due to thermal expansion during operation.
Particle generation is a serious problem in magnetron plasmas, particularly in magnetron plasmas employed in the production of semiconductor devices and display technology, primarily because of the sensitivity of the final product to particle contamination, and the sensitivity of each film used in manufacturing the final product. In typical magnetron sputtering operations, film build-up on the internal surfaces of the tool is continuous. Eventually, a significant amount of film is re-deposited onto the target surface, which film tends to undergo flaking, peeling and separation from the target surface due to the inherent stress of the film and the material stress from thermal expansion and contraction of the target.
As a consequence of the peeling and separation of re-deposited sputtered particles from the target surface, contaminant particles are introduced into the sputtering plasma, greatly reducing product throughput and increasing yield loss.
It is known that de-lamination of re-deposited particles may be reduced through surface texturing. Surface texturing is often accomplished by grit blasting the target surface with abrasive material, or through application of arc-spray to achieve surface roughness values exceeding 200 microinch RMS values. One disadvantage of such texturing techniques is that they introduce foreign material into the sputter target. Since sputter surfaces are made of high purity material, introduction of foreign material into the target surface is a major contamination issue. Moreover, abrasive media is often insulating or semi-conducting and embedded media can cause charge accumulation leading to arcing behavior.
Surface texturing may also be accomplished by machining or knurling to make fine regular cuts into the target surface. One disadvantage of this approach is that the deposited layer may bridge the gaps and there is no inhomogeneity to the pattern.
Another prior approach to the sputtering particle contamination problem comprises forming grooves for trapping re-deposited sputter material along the peripheral portion of the target, as disclosed in U.S. Pat. No. 6,117,281. With this approach, a high number of electrons remain in the grooves due to the cosine component of the magnetic force, thus preventing the flow of electrons to the race track formed in the target. However, this approach does not contemplate forming thermal expansion slots proximate the center of the target surface to reduce separation of the re-deposited particles.
Japanese Patent Publication No. 61291964 A describes an approach comprising forming grooves along the entire target surface so as to absorb thermal expansion and prevent thermal deformation of a resin target. However, this approach does not specifically address the contamination problem of peeling and separation of re-deposited sputtered particles from the target surface, and there is no inhomogeneity to the pattern.
Therefore, there remains a need in the art of sputter target assemblies for a method of providing a sputter target which reduces the inherent stress from particle re-deposition in a non-homogeneous manner and without introducing foreign material to the sputter target.